In many industries and applications, a common method for constructing a structure is to form cement support structures and attach thereto a covering or lining layer of metal, such as steel. In at least some applications, there is a need to inspect the cement support structures that are hidden behind steel (and other metal) plates and tubes. For example, in the case of gas and oil industries, steel/cement structures are commonly utilized to form pipelines and conduits for gas and oil wells. That is, after the well is drilled, a cement layer is disposed on the rock face of the well (to provide a grout) and a steel (or other metal) casing is positioned on the cement to provide to define the interior of the well. In these cases, there is a critical need for inspection of these components to verify the integrity of the grout and the casing, as a failure in either of these can lead to contamination of the environment (e.g., contamination of the water table). Ideally, such an inspection would include determining (1) whether grout has de-bonded from the casing, (2) location of cracks, voids, fissures, or other anomalies and defects in the grout and the casing, (3) the shape and position of the anomalies and defect, (4) a depth or thickness of grout, and (5) presence of water or other fluids between the rock face and the casing.
Ultrasonic inspection methods have been used for pipeline and wellbore inspections for some time. An example of one commonly used inspection system is described in U.S. Pat. No. 5,717,169 to Liang et al., issued Feb. 10, 1998. However, methods for examining cement-type structures generally require access to two sides of the cement structure, a near impossibility in the case of oil and gas wells. As such, ultrasonic inspection methods in these industries are generally limited to an examination of the steel casing for flaws. Another difficulty in the case of ultrasonic inspection of oil and gas wells is that de-bonding of the cement from the steel casing can prevent penetration of acoustic waves from the steel to cement, because of air gap in between. A further problem is that the density and porosity of grout materials, such as cement, is highly variable and strongly affects the attenuation. For example, higher porosity results in more signal being lost through the porous material.